The general process of image capture and display is illustrated in simplified form in FIG. 1. Light from the original image 10 is received by the image sensor 20 (usually a charge coupled device, or CCD) and converted into an electrical signal (X'). A digital signal processor (DSP) 30 is typically used to enhance signal X', including color correction, image enhancement, contrast control, white balance, luminance processing, and chrominance processing. The enhanced signal output (X) of DSP 30 is then gamma corrected by a gamma correction circuit 40, to provide an output (Y) which is inversely compensated for the well known non-linearity characteristic of the cathode ray tube (CRT) display monitor 50. The displayed image (Z) is a visual representation of original image 10.
The aforementioned non-linearity characteristic of a typical cathode ray tube is depicted in FIG. 2A. The relationship between signal voltage (Y) and light output (Z) may be described as a power law, as follows: EQU Z=Q*Y.sup..gamma. Equation ( 1)
where Z is the linearized light output of the CRT, Q is a constant, Y is the gamma corrected input signal to the CRT, and .gamma. is a constant.
Thus, it can be seen from Equation (1) and FIG. 2A that the light output (Z) increases more rapidly than the input signal (Y). The effect of this CRT non-linearity is to stretch out changes in luminance for the high--light intensity signals, and to compress these changes for the low-light intensity signals. To compensate for this effect, the inverse of Equation (1) has typically been applied to the CRT input signal by means of a gamma correction circuit, (e.g. circuit 40 of FIG. 1) as indicated in FIG. 2B, and in Equation (2), as follows: EQU Y=k*X.sup.1/.gamma. Equation ( 2)
where Y is the output of the gamma correction circuit, k is a constant, X is the input to the gamma correction circuit, and 1/.gamma. represents the gamma correction factor. A typical value for gamma is known to be 2.2, so that the gamma correction factor (1/.gamma.) becomes 0.45.
The combined effect of gamma correction and CRT non-linearity results in a linearized system, as indicated in FIG. 2C, and in Equation (3), as follows: EQU Z=k*X Equation (3)
where Z is the linearized light output of the CRT, k is a constant, and X is the input to the gamma correction circuit.
While the above described gamma correction technique can compensate for CRT non-linearity, it is not adequate for correcting other image capture and display problems. These problems include non-linearities in the image sensor (20 of FIG. 1), the DSP (30 of FIG. 1), and the light sensitivity of the human eye. Therefore, to provide the most accurate visual display possible, a more comprehensive type of gamma correction is required.
In the prior art U.S. Pat. No. 4,786,968, entitled "Gamma Correction of Digital Video Data by Calculating Linearly Interpolated Gamma Correction Values" (M. Kutner Nov. 22, 1988), only gamma values approximately equal to 1 were considered.
In U.S. Pat. No. 4,833,527, entitled "Luminance Signal Forming Circuit" (T. Kondo May 23, 1989), a very complex circuit was used to improve the reproducibility of low signal-to-noise ratio luminance signals.
In U.S. Pat. No. 5,087,966, entitled "Digital Gamma Correction" (V. Harradine Feb. 11, 1992), a digital technique was developed to simulate the typical analog gamma correction.
In U.S. Pat. No. 5,089,890, entitled "Gamma Correction Device" (T. Takayama Feb. 18, 1992), multiple gamma correction values were computed for application to corresponding input signal levels, requiring complex circuitry and time-consuming computations.
It is an object of the present invention to provide a simple hardware implementation of a digital gamma correction circuit which will address the full range of non-linearities in a typical image capture and display system.
It is a further object of the present invention to provide a three-stage type of gamma correction circuit, corresponding to low, medium, and high levels of signal intensity, in order to compensate for the sensitivity characteristics of the human eye, as well as for the aforementioned hardware non-linearities.